Referring now to FIG. 1, there is shown a exemplary prior art planar star coupler 10 (or one input-to-n output signal coupler) comprising a Free Propagation Region (FPR) 12, an input waveguide 13, and a plurality of “i” output waveguides 14a-14i for use with lightwave transmission. The FPR 12 comprises a first interface 15 having a partial cylindrical shape on which an output of the input waveguide 13 is terminated, and a second opposing interface 16 having a partial cylindrical shape. The second opposing interface 16 has a radius related to that of the first interface and is spaced apart at a distance R from the first interface 15. The second opposing interface 16 terminates the inputs to the plurality of “i” output waveguides 14a-14i. Inputs 18a-18i to the plurality of “i” waveguides 14a-14i, respectively, have their axis spaced apart from the axis of any one of the adjacent output waveguides 14a-14i by a distance “t”. The input waveguide 13 has a width W1, and an input of each one of the output waveguides 14a-14i has a width W2 at the second interface 16, and then tapers to a width W3.
The FPR 12 of the planar star coupler 10 is essentially a slab waveguide, and tends to guide light in a vertical plane (in a direction out of the paper in FIG. 1) but allows light to travel unguided in the horizontal plane (in the plane of the paper of FIG. 1). In operation, an optical signal is launched into the input of the waveguide 13 and propagates in the input waveguide 13 to the output at the first interface 15 of the FPR 12. Upon entering the FPR 12, the optical signal diffracts freely in the horizontal plane. As the light reaches the second interface 16, each of the output waveguides 14a-14i is placed to accept a portion of light incident upon its input 18a-18i, respectively. The fraction of light that overlaps the inputs 18a-18i of the waveguides 14a-14i, respectively, then gets coupled into the respective waveguides 14a-14i. The light pattern at the second interface 16 can be defined as a Fourier transform of the light pattern at the input waveguide 13.
Referring now to FIG. 2, there is shown a mode profile 20 of an input signal at the first interface 15 of the FPR 12, and a mode profile 22 at the second interface 16 of the star coupler 10 of FIG. 1. In a conventional waveguide, the mode profile is nominally Gaussian shaped and, accordingly, the diffracted pattern has a nominal Gaussian profile as is shown for the mode profiles 20 and 22.
Referring now to FIG. 3, there is shown a diagram of a first circle 25 and a second inner circle 26, called the Rowland circles, which illustrate the technique of forming the FPR 12 of FIG. 1. The first circle 25 has a radius r, where a portion of the circumference thereof forms the arc of the second interface 16 of the FPR 12. The second inner circle 26 has a diameter R which may or may not (depending on the design) intersect the center and the circumference of the first circle where the second interface 16 is located as is shown in FIG. 3. A portion of the circumference of the second inner circle 26 opposite the second interface 16 forms the arc of the first interface 15 of the FPR 12.
Referring now to FIG. 4, there is shown a function of a mode profile 22 (shown in FIG. 2) of light found at the second interface 16 of the star coupler 10 of FIG. 1, and periodic mode profiles 24a-24i of light at the inputs 18a-18i of the output waveguides 14a-14i, respectively, at the second interface 16 of the FPR 12 of FIG. 1. The X axis is shown in units of microns, whereas the Y axis is shown in normalized values of intensity. With the mode profiles 24a-24i, the light incident on the portion of the second interface 16 where there is no input to any one of the waveguides 14a-14i is not coupled and is lost by reflection and leakage and, therefore, produces an insertion loss.
It is desirable to provide a planar star coupler with a reduced insertion loss from that found in prior art star couplers.